Lithium batteries, especially lithium-polymer batteries, become dangerously volatile when discharged too much. For this reason, any electronic devices powered by lithium must have some ability to automatically shut off if the voltage goes too low (this value is usually around 3V). This is why modern lithium-powered cordless drills shut off suddenly when the battery gets low, unlike NiMh or NiCad batteries which you could actually fully discharge by running the drill motor until it stopped turning. If lithium batteries are not disconnected electronically well before that happens, they're likely to catch fire.